Vanillin is a key additive to food products, perfumery, beverage and an intermediate in pharmaceutical industry. The world production of synthetic vanillin is estimated to be over 3000 tones per annum. With the current growth rate (5.75% annually) the global demand is likely to make an exponential growth. Vanillin is naturally obtained from Vanilla planifolia Andrew (Orchidaceae) pods. Vanilla flavour extracted from cured beans of the vanilla plant (Vanilla planifolia Andrew) is used as a flavouring agent in food and currently has high demand. The traditional method of production of natural vanillin is cumbersome as it involves cultivation of plants in restricted climatic regions, hand pollination with limited success and the tedious processes for curing and extraction of products from the beans. This ‘natural’ vanillin, can cover only 1% of the global demand and is costly (US$4,00.00-500.00 per kilogram) [Virginia Commonwealth University News, 14.04.2003, vcunews@vcu.edu]. It is estimated that even to cater to 5% of global demand of vanillin 2.39 Lakh hectare of land has to be brought under cultivation causing deforestation of huge tract of virgin forest [www.tidco.com/tidcodocs/tn/Opportunities/Vanillin.doc]. Apart from that natural calamity can cause havoc in vanilla cultivation. In 2001 a devastating hurricane in Madagascar the world's biggest vanilla producer destroyed about 35% of the crop and 15% of the stocks in storage, sky rocketing the price from US$20.00-US$40.00 to US$200.00-US$230.00 a kilogram [www.foodnavigator.com/newsnews-NG.asp].
The recent focus on vanillin production is centered on isolation of ferulic acid from agro-waste and its subsequent conversion to vanillin. Recently biotechnological approaches have been initiated to achieve this target. Some progress is being made in developed countries [P. Bonnarme, G. Ferm, A. Durand; Ind. Perfumer, 45(1), 63-67, 2001] to prepare vanillin through a biotechnological route from ferulate-moiety containing agricultural waste. Here ferulic acid is first isolated from its source, which is converted to vanillin and vanillic acid using microbes [Synthesis of vanillin from a carbon source; John W. Frost, U.S. Pat. No. 6,372,461 and references cited]. Use of glucose instead of ferulic acid as feed-stock results in microbial conversion to vanillic acid. The latter is reduced to vanillin by an aryl-aldehyde dehydrogenase enzyme released from a genetically manipulated E. coli microbe [Synthesis of vanillin from a carbon source; John W. Frost, U.S. Pat. No. 6,372,461]. The biotechnological processes require multi-steps, heavy investment and stringent control to dictate the specific enzymes to work, and have high risk of mutation or inhibition of growth of microorganisms in presence of metabolites and are slow. Therefore, there is a scope to develop chemical methods that will be able to convert ferulate moiety of agricultural waste to vanillin in minimum chemical steps in an environmentally benign way.
Any new process aimed at preparation of vanillin today must concern itself for use of renewable resources and environmentally-friendly chemicals and solvents. In this case the ideal raw material is ferulate moiety containing agro-wastes such as rice straw, which is cheap and abundant, hydrogen peroxide as the oxidizing agent and water as solvent.
The hitherto relevant known methods for preparation of vanillin are:
A reference may be made to the guaiacol method [Esposito L. et.al., “Vanillin” Kirk-Othmer Encyclopedia of Chemical Technology; Vol: 24, 812-814, 1997; Fourth Ed., Kroschwitz, J. I., Howe-Grant, M., Ed., Wiley: New York]
Vanillin is now industrially prepared in multi-steps using guaiacol as the starting material. Guaiacol itself is prepared from catechol. Catechol is prepared from phenol a crude-oil based chemical.
The drawback of the process is that it involves multi-steps and non-renewable crude oil based chemicals.
Another reference may be made to Vanillin from lignin [Esposito L. et.al., “Vanillin” Kirk-Othmer Encyclopedia of Chemical Technology; Vol: 24, 812-814, 1997; Fourth Ed., Kroschwitz, J. I., Howe-Grant, M., Ed., Wiley: New York]
This industrial method of preparation of vanillin is based on lignin as raw material. In this method lignin from cellulose industry is treated with alkali at elevated temperature in presence of oxidants. The vanillin formed is separated from by-products such as acetovanillin, 4-hydroxy-3-methoxy acetophenone and few other compounds.
The drawback of the process is that in spite of use of renewable lignin as raw material the process generates 160 tones of caustic wastes per ton of vanillin. This volume of waste is a threat to the environment.
Yet another reference may be made to Vanillin from ferulic acid [W. A. Herrmann, T. Weskamp, J. P. Zoller, R. W. Fischer; J. Mol. Catal., 153, 49-52, 2000]
In this method trans-ferulic acid is converted to vanillin using hydrogen peroxide in presence of methyl trioxorhenium complex as catalyst.
The drawback of the method is that methyl trioxorhenium complex is costly and short-lived in presence of water. On the other hand commercial hydrogen peroxide (ca. 30%) contains water. The reaction has to be carried out in a good amount of water-trapping agent magnesium sulphate. The reaction can be carried out using methyl tributyl ether as solvent, which is toxic and expensive. Under such a stringent condition the reaction remains only of academic interest.
Yet another reference may be made to Vanillin from alkali lignin by green chemical method [Zhou Qiang; Chen Zhonghao; China Patent No. CN 1285395; 2001]
In this method vanillin is prepared from alkali lignin by catalytic oxidation reaction with recovery of catalyst and solvent.
The drawback of the method is that lignin has to be separated with alkali prior to the oxidation reaction resulting in multiple steps.
Yet another reference may be made to Vanillin from alkali lignin by nitrobenzene oxidation method [Kozlov I. A., Kuznetsov B. N., Gogotov A. F., Rybal Chenko T. So Ran; Russian patent No. RU2179968, 2002] In this method lignin is oxidized by alkaline nitrobenzene.
The drawback of the method is that it uses nitrobenzene a toxic compound and the method also produces syringaldehyde as the major product.
Considering the environment-polluting non-renewable petroleum crude based chemicals employed in the existing methods there is a great scope for inventing environmentally-friendly chemical processes that can use renewable raw materials for the synthesis of vanillin. It is also important to recycle chemicals and solvents in one hand for optimum utilization and on the other to answer the demand of the environmental considerations. The use of renewable starting materials such as agrowastes do not demand extra virgin land and water for cultivation.
Hydroxycinnamic acids, particularly ferulic acid (4-hydroxy-3-methoxy cinnamic acid, occurs widely in the cell walls of graminaceous plants. All the cell walls of sugarcane and rice straw contain 5-15% hydroxycinnamic moieties based on total lignin. The conversion of ferulate (hydroxycinnamate) moiety into vanillin is of current global interest. But the interest is confined to biotechnological methods only. The biotechnological processes are slow, require heavy investment and stringent control to dictate the specific enzymes to work, and have high risk of mutation or inhibition of growth of microorganisms in presence of metabolites. Therefore, it is very much necessary to develop clean chemical methods to convert ferulates of agrowastes such as rice straw to vanillin eliminating the drawbacks of other methods cited above. The present invention answers all the above concerns by using rice straw as the renewable raw material, water as solvent and hydrogen peroxide as the oxidizing agent along with a catalyst and co-catalyst both of which are cheap, recyclable and stable in presence of water. The synthesis of vanillin can be done by this invention in a single simple step. All the chemicals and solvent can be recycled except hydrogen peroxide. Hydrogen peroxide is consumed in the reaction with release of water as the by-product.